The primary objective is to develop new inorganic scintillators for detecting 511 keV photons with exceptional detection efficiency (similar to BGO), response time (1 ns), and energy resolution (3% fwhm). Such a scintillator would greatly increase the effective sensitivity of positron emission tomography (PET) by (1) providing time-of-flight information to the reconstruction process, (2) by rejecting tissue-scattered photons by pulse height selection rather than by inter-plane septa, (3) by rejecting random coincidences to the maximum extent possible, and (4) by a 40-fold reduction in deadtime compared with LSO. A timing resolution of 200 ps fwhm would increase the effective sensitivity by a factor of six for images of the human brain and even more for the thorax. The elimination of inter-plane septa will increase the solid angle efficiency by an additional factor of two or three. Additional advantages include simultaneous emission and transmission measurements, and reduced axial blurring. We have studied available scintillators and the reasons why none can provide the speed and brightness necessary for the full promise of septaless, time-offlight PET. After reviewing radiative recombination processes in semiconductors, we have identified an unexploited scintillation mechanism that would enable the development of a new class of scintillators and revolutionize PET. Specifically, if a direct-gap semiconductor is codoped with (1) donor atoms to provide a band of electrons near the bottom of the conduction band, and (2) other impurity atoms to trap efficiently the holes produced in an ionization event, the holes and electrons will combine radiatively to produce fast (ns), bright scintillation. We have experimentally verified this principle by decreasing the primary decay times of CdS(Te) from 3 mu s to 3.5 ns by codoping with indium to provide a donor band of electrons. To identify semiconductors with better stopping power for 511 keV photons, we will continue to improve our band structure calculations to guide the selection of unexplored heavy semiconductors and dopants. Quantities to be calculated include (1) density and photoelectric stopping power, (2) band gap, (3) effective electron and hole masses, and (4) energy levels of various donor and accepter atoms. Candidate materials will be synthesized as powders or crystals and tested using our pulsed x-ray system and the same techniques used in the previous funding period. A secondary objective of this research is the identification of dense, heavy-atom crystals that would be good hosts for the widely used scintillation activator ion Ce3+. The goal is to identify alternatives to LSO with improved speed, light output, and cost.